The present invention relates to a film having optical functions such as anti-glare properties. More specifically, the invention relates to a film with excellent optical characteristics which is suitable as an anti-reflection film on the surface of various displays of word processors, computers and TV sets, polarizing plates for use in liquid crystal display devices, lenses of sunglasses composed of transparent plastics, lenses of vision-correcting eyeglasses, optical lenses such as camera finder lenses, covers for various gauges, and window glasses of automobiles and electric trains.
Transparent base plates of glass, plastic or the like are used for displays of curve mirrors, rear view mirrors, goggles, window glasses, displays for personal computers and word processors, and various other displays for commercial use. One may see objects or recognizes visual information, such as characters or graphics, through these transparent base plates, or may observe images from a reflection layer through the transparent base plate of the mirror. In so doing, one may feel it difficult to discern the necessary visual information located in the transparent base plate, because the surface of the transparent base plate reflects light.
Techniques for preventing the reflection of light include a method of coating an anti-reflection coating on the surface of a glass or plastic material, a method of providing a super thin film of MgF.sub.2 or the like or a vacuum deposited metal film about 0.1 .mu.m thick on the surface of a transparent base plate of glass or the like, a method of coating an ionizing radiation curing resin on the surface of a plastic lens and forming a film of SiO.sub.x or MgF.sub.2 on the coating by vacuum deposition, and a method of further forming a coating with a low refractive index on a cured film of an ionizing radiation curing resin.
The above-mentioned thin film of MgF.sub.2 about 0.1 .mu.m thick formed on glass will be described in further detail. This thin film as an anti-reflection film is required to prevent reflection of light completely and allow 100% transmission of light when incident light falls vertically on the thin film in the air. With a specific wavelength designated as .lambda..sub.0, the refractive index of the anti-reflection film for this wavelength as n.sub.0, the thickness of the anti-reflection film as h, and the refractive index of the base plate as n.sub.2, the relations of the following Equations 1 and 2 must hold to fulfill the above requirements, as have been already known (Science Library, Physics=9 "Optics", pp. 70-72, 1980, Science Ltd.): EQU n.sub.0 =(n.sub.2).sup.1/2 (Equation 1) EQU n.sub.0 h=.lambda..sub.0 +L /4 (Equation 2)
The refractive index of glass n.sub.2 =1.5, the refractive index of MgF.sub.2 n.sub.0 =1.38, and the wavelength of incident light .lambda..sub.0 =5500 .ANG. (reference) are known. Substituting these values into Equation 2 yields about 0.1 .mu.m as the optimal thickness h of the anti-reflection film.
Equation 1 tells that an effective way of preventing reflection of light 100% is to select a material in which the refractive index of a coating on the upper layer side is close to the square root of the refractive index of the lower layer laid under the coating.